Address review feedback

src/onTTFB.ts

@@ -69,26 +69,22 @@ export const onTTFB = (onReport: ReportCallback, opts?: ReportOpts) => {
     if (navEntry) {
       const responseStart = navEntry.responseStart;
 
-      // In some cases the value reported is negative or is larger
-      // than the current page time. Ignore these cases:
+      // In some cases no value is reported by the browser (for
+      // privacy/security reasons), and in other cases (bugs) the value is
+      // negative or is larger than the current page time. Ignore these cases:
       // https://github.com/GoogleChrome/web-vitals/issues/137
       // https://github.com/GoogleChrome/web-vitals/issues/162
-      if (responseStart < 0 || responseStart > performance.now()) return;
+      // https://github.com/GoogleChrome/web-vitals/issues/275
+      if (responseStart <= 0 || responseStart > performance.now()) return;
 
-      // If the navigation entry's `responseStart` value is 0, ignore it.
-      // This likely means the request included a cross-origin redirect, and
-      // the browser has removed timing info for privacy/security reasons.
-      // See: https://github.com/GoogleChrome/web-vitals/issues/275
-      if (responseStart > 0) {
-        // The activationStart reference is used because TTFB should be
-        // relative to page activation rather than navigation start if the
-        // page was prerendered. But in cases where `activationStart` occurs
-        // after the first byte is received, this time should be clamped at 0.
-        metric.value = Math.max(responseStart - getActivationStart(), 0);
+      // The activationStart reference is used because TTFB should be
+      // relative to page activation rather than navigation start if the
+      // page was prerendered. But in cases where `activationStart` occurs
+      // after the first byte is received, this time should be clamped at 0.
+      metric.value = Math.max(responseStart - getActivationStart(), 0);
 
-        metric.entries = [navEntry];
-        report(true);
-      }
+      metric.entries = [navEntry];
+      report(true);
 
       // Only report TTFB after bfcache restores if a `navigation` entry
       // was reported for the initial load.

test/e2e/onTTFB-test.js

@@ -185,30 +185,28 @@ describe('onTTFB()', async function() {
     assert.strictEqual(ttfb2.entries.length, 0);
   });
 
-  it('ignores navigations with no responseStart timestamp', async function() {
-    await browser.url('/test/ttfb?responseStart=0');
+  it('ignores navigations with invalid responseStart timestamps', async function() {
+    for (const rs of [-1, 0, 1e12]) {
+      await browser.url(`/test/ttfb?responseStart=${rs}`);
 
-    await domReadyState('complete');
+      await domReadyState('complete');
 
-    // Wait a bit to ensure no beacons were sent.
-    await browser.pause(1000);
+      // Wait a bit to ensure no beacons were sent.
+      await browser.pause(1000);
 
-    const loadBeacons = await getBeacons();
-    assert.strictEqual(loadBeacons.length, 0);
+      const loadBeacons = await getBeacons();
+      assert.strictEqual(loadBeacons.length, 0);
 
-    // Test back-forward navigations to ensure they're sent, even if the
-    // initial page TTFB value is ignored.
-    await stubForwardBack();
+      // Test back-forward navigations to ensure they're not sent either
+      // in these situations.
+      await stubForwardBack();
 
-    const ttfb = await getTTFBBeacon();
+      // Wait a bit to ensure no beacons were sent.
+      await browser.pause(1000);
 
-    assert(ttfb.id.match(/^v3-\d+-\d+$/));
-    assert.strictEqual(ttfb.value, 0);
-    assert.strictEqual(ttfb.name, 'TTFB');
-    assert.strictEqual(ttfb.value, ttfb.delta);
-    assert.strictEqual(ttfb.rating, 'good');
-    assert.strictEqual(ttfb.navigationType, 'back-forward-cache');
-    assert.strictEqual(ttfb.entries.length, 0);
+      const bfcacheBeacons = await getBeacons();
+      assert.strictEqual(bfcacheBeacons.length, 0);
+    }
   });
 
   describe('attribution', function() {